There is an irreversible trend in today's wireless communication systems that more and more communication systems of different protocols and various working frequencies are integrated into one increasingly compact physical unit. Such trend impacts not only mobile terminals but also network infrastructure equipment such as base stations and wireless routers. In a mobile unit, such as a mobile phone or a laptop computer, multiple wireless services, including GSM, UMTS, Wi-Fi, LTE, GPS and Bluetooth, coexist in a very compact space. The radio frequency interference among the systems that operate at the same or adjacent frequency bands can seriously affect the quality of service. Meanwhile, in a today's wireless base station, antennas for 2G (GSM), 3G (UMTS), 4G (LTE) as well as Wi-Fi wireless communication systems must coexist in a close vicinity, which inevitably create radio frequency interference to each other through antennas since these frequency bands are very close to each other.
Although there are many preliminary attempts from industry and academic communities to avoid such interference by adding filters and increasing spatial isolation between two antennas as far as possible, the interference among the systems operating at adjacent frequencies cannot be sufficiently suppressed if the spatial isolation is not sufficient. The coexistence interference issue among radio transceivers and antennas becomes increasingly important as the size of an integrated system decreasing and the number of wireless systems increasing.
Current solutions for reducing the interference can be divided into three categories: signaling-based solutions that require coordination between the collated transceivers, active interference suppression solutions that need complex active circuitry and control algorithm, and antenna isolation enhancement solutions by passive networks. However, none of them works for two radio systems working in two adjacent frequency bands.